Braking systems using electromechanical actuators (EMAs) have been considered as alternatives to conventional hydraulic braking arrangements. In one previously disclosed aircraft braking arrangement using EMAs, a plurality of EMAs are mounted on a brake carrier housing in an annular pattern about the axis of wheel rotation. The brake carrier housing is fixed to a torque tube having stator disks of a brake disk stack attached thereto. Rotor disks of the brake disk stack, which extend between the stator disks attached to the torque tube so that rotor and stator disks alternate, are fixed to and rotatable with the wheel that rotates about an axis. The EMAs are selectively energized in response to a braking command, causing a motor-driven, reciprocating actuator piston (“ram”) to extend and engage with a pressure plate positioned on one end of the brake disk stack so as to compress the brake disk stack and retard wheel rotation. One EMA-based braking system is disclosed U.S. Pat. No. 6,530,625, titled “Electrically Actuated Brake With Vibration Damping,” the entire contents of which are herby incorporated by reference.
The commercial need that one aspect of the present invention is intended to solve is how to implement a parking brake function on an electromechanically actuated aircraft brake which effectively addresses the following design constraints: (1) the parking brake should effectively hold without requiring the continuous application of power; and (2) the failure mode of the park brake should be such that loss of power does not automatically actuate the park brake. Item (1) above is desirable because only limited-capacity battery power may be available while the aircraft is parked and the engines turned off. The total energy required by the park brake to hold the aircraft should be a small percentage of the battery's energy storage capacity so as not to discharge the battery. The constraint described in item (2) above arises from safety considerations. Loss of power to the electric brake system should not result in uncommanded braking. Uncommanded braking is considered a more serious failure mode than loss of braking. Attempting to implement a park brake function by simply locking the brake actuators in a fixed position does not solve the problem because of the effects of differential thermal expansion in the brake heat stack. As the heat redistributes, and differential thermal expansion takes place, the clamping force resulting from a locked actuator changes.